My guess is that it doesn't have enough drag in 1st and 2nd gear to reach full boost.

You exhaust gas and pressure comes in proportion to the amount of work your car has to do. The higher the gear, the more exhaust gas pressure since the motor has to work more. I'm over simplifying this here but just imagine imagine blowing on a pin wheel... if you blow on it gently it will spin a little bit, but if you blow with all of your might then it spins really hard. In 1st and 2nd you may not be blowing hard enough to make it spin all the way.

If you're using the cobb then they have maps for those who are not able to reach full boost... might want to send them an email.